The present invention relates to a method and apparatus for forming a heating element to have a predetermined resistance and, more particularly, for forming a corrugated heating element to a predetermined resistance and a predetermined overall length, regardless of variations in the resistance per unit length of the pre-formed ribbon, by monitoring the ribbon resistance and varying the amplitude of the corrugations based that per unit length resistance.
Resistive heating elements are commonly used to generate heat based on the flow of electric current. The amount of heat that the element generates, in terms of watts, is determined by the resistance it offers to the flow of current. The resistance is determined by the material from which the element is made and on the element's dimensions. For a given material, resistance is directly proportional to the element's length and inversely proportional to its cross sectional area. The wattage of the heat produced is equal to the element's resistance, in Ohms, multiplied by the square of the current, in amps, flowing through the element. The wattage can also be calculated in terms of the voltage drop across the element, where it is the square of the voltage across the element divided by its resistance.
Heating elements find application in many industrial processes and are also used in the home. In some applications, it is important to be able to control the characteristics of the element and the energy produced thereby. For example, when a heating element is placed beneath a protective glass-ceramic cover, such as those found on many cooking surfaces, the wavelength at which the element radiates is important because such covers are not transparent to all wavelengths. To maximize the amount of radiation that passes through the cover a heating element must be designed to emit the majority of its energy at wavelengths to which such covers are transparent.
Adjusting the amount of heat produced by a heating element, i.e., adjusting the wattage, is accomplished by changing the current through the element, or by changing the resistance of the element The resistance is changed by varying the length or cross-sectional area of the element, by forming the element from materials having different resistivities. In practice, however, certain materials are used for heating elements because of their durability, price, and other qualities and thus the resistivity of a heating element will be fixed for many applications. Likewise when the heating element is required to produce a certain amount of power at a given voltage, it may not be practicable to control the current. Therefore, the desired heat output is most easily obtained by adjusting the resistance of the element, and this in turn is done by properly selecting the length and cross sectional area of a resistive heating element.
As summarized above, the wavelength of the energy produced by the heating element depends on its operating temperature. The wavelength is generally specified to meet the wavelength-dependent heat opacity and reflectivity characteristics of the structures that will surround the heating element. The operating temperature, in turn, is a function of the dimensions of the heating element and its wattage. It is a function of the dimension because a large element dissipating a given wattage will generally have a lower operating temperature than a small element dissipating the same wattage, assuming the two are made of the same material.
The wattage of the heating element is generally a specified parameter, as this is heat generating capacity of the element. Wattage depends on resistance and voltage and, in general, voltage is a predetermined quantity. Therefore, from the required wattage and voltage, the resistance necessary to generate that wattage is calculated. Resistance is a function of the material and the dimension of the element. The material is selected based on cost, operating temperature, operating environment, and special considerations, such as corrosive atmosphere, vibration and the like. The cross-sectional area of the element is generally pre-specified according to the size and mounting structure of the heating element. Therefore, after the resistance is calculated the designer determines the length of element needed to produce a given resistance.
Once the necessary length of material has been obtained, the designer must arrange the element in some configuration, within a defined area in a heating unit, so that it will produce a given distribution of heat. The distribution may be even or may vary in a particular way. Unfortunately, the best pattern for heat distribution purposes may be shorter than the length of the heating element needed to produce the desired power output. The designer must therefore compromise the layout that would have been optimal from the standpoint of heat distribution alone, and engage in the generally time consuming practice of finding alternate but still acceptable pathways for laying out the fixed length of heating element.
Assuming that the alternate layout pattern can be found, there is an additional problem arising from variations in the resistance per-unit length of the material. The problem is that the length calculated for achieving the desired resistance, and hence wattage, does so only for a material having the exact resistance per unit length for which the length was calculated. However, if a new batch of heating element material is procured having a resistance per unit length, for example, that is 1% lower than the material for which an acceptable layout was found, then a heating element made from the new material will have 1% lower resistance than the prior one. A 1% variation in wattage, however, may be unacceptable.
A solution known in the prior art would be to increase the length of the element formed of the new material by 1%. There are at least two problems arising from this solution. One is the time required to measure the resistance per unit length of each batch of heating element material and then adjust the length accordingly. The other is that changing the length of the finished element by 1% may cause mounting problems. A one percent change in overall physical dimension could cause fitting or clearance problems or, if mounting points were at specific locations on the element patterns, could cause misalignments.